Method for fusing carbon electrodes and insulating spacers used in electric overvoltage arresters

ABSTRACT

An improved process for fusing a carbon electrode to an insulating spacer is provided for use in making a component for electric overvoltage arresters. The improved process involves heating an insulating spacer, a carbon electrode and a bonding composition used to fuse the electrode to the spacer in an inert atmosphere substantially free from oxygen until the bonding composition is liquefied, setting the position of the electrode in the insulating spacer while the bonding composition is in a liquid condition, and then allowing the bonding composition to harden so that the electrode is permanently secured within the spacer.

United States Patent Patel 15] 3,655,484 [451 Apr. 11, 1972 [72]Inventor: Vinayak K. Patel, Chicago, Ill.

[73] Assignee: Cook Electric Company, Morton Grove,

Ill.

[22] Filed: Oct. 15,1969

[21] Appl.No.: 866,495

3,420,683 1/1969 Ikeda ..106/49 1,340,812 5/1920 Bernard ..l56/294X1,164,739 12/1915 Mershon ...156/293 X 1,440,003 12/ l 922 Bultemann......156/293 2,144,558 1/1939 Bahls ..106/46 X 2,855,581 10/1958 Freedom...3,000,092 9/1961 Scuro Primary Examiner-Carl D. Quarforth AssistantExaminer-Gary G. Solyst Attorney-Mason, Kalehmainen, Rothbum & Wyss [57]ABSTRACT An improved process for fusing a carbon electrode to aninsulating spacer is provided for use in making a component for electricovervoltage arresters. The improved process involves heating aninsulating spacer, a carbon electrode and a bonding composition used tofuse the electrode to the spacer in an inert atmosphere substantiallyfree from oxygen until the bonding composition is liquefied, setting theposition of the electrode in the insulating spacer while the bondingcomposition is in a liquid condition, and then allowing the bondingcomposition to harden so that the electrode is permanently securedwithin the spacer.

4 Claims, 3 Drawing Figures INSULAT/NG SPACER I INSERT CARBON 52507-2005INTO SPA GER /N$(/LA7'/NG SPA CER CONTAIN/7V6 CARBON E1- ECI'RDDEINSULATING SPACER AA? [A ECI'RODE vwru BONDING COM/ 06'! T/ON HEAT IN ANMIL-k7 ATMUSPHERfi sussmlvr/nur F/lff F POM axraav SET POSITIONDFELEC'TROOE l/V IABl/LA TIA/6 SPACE)? BONDED IMSlll/lTl/VG' SPACER ANDflIOI'PODEwn-w 557 AIR GAP ,FIAI/JHED INSULA 7' INS \SPA C51? BONDED T0EL 5c TRODE METHOD FOR FUSING CARBON ELECTRODES AND INSULATING SPACERSUSED IN ELECTRIC OVERVOLTAGE ARRESTERS BACKGROUND OF THE INVENTION Thisinvention relates to an improved process for making electric overvoltagearresters used to protect telephone and other electrical transmissionlines from seriouson-line voltage surges. More specifically, theinvention concerns a method for fusing the carbon electrode andinsulating spacer components of a spark gap type surge arrester toimprove the in-service reliability of the resulting arresters assembledfrom such components.

Typically, spark gap type overvoltage or surge arresters are constructedfrom a pair of spacer electrodes, an insulating spacer or sleeve whichsurrounds one of the electrodes and a suitable housing to hold theelectrodes in a spaced arrangemeritv The spark gap formed between theelectrodes functions as a protective device for the transmission lineconnected to the arrester. Under normal conditions, the spark gapinsulates the arrester from the line and prevents the flow of leakagecurrent through the arrester. When a lightning strike or some other highvoltage surge occurs in the line, however, the air gap between theelectrodes momentarily breaks down, thus permitting the flow of surgecurrent through the arrester to ground;

Consequently, it is desirable that overvoltage arresters have a highcurrent carrying capability, closely controlled firing or breakdownvoltages, firing voltages independent of environmental conditions, fastfiring times, and a repeated surge capability or long life. This latercharacteristic, namely, long life, is particularly important when anovervoltage arrester is intended for use in telephone line protectiveequipment where high reliability service is desired. One recurringproblem existing in current overvoltage arresters, which employ a pairof spaced carbon electrodes and an insulating spacer which surrounds oneof the electrodes and fixes the gap space between electrodes, is thefrequent failure of such arrester units after relatively few firings. Ofcourse, one explanation for these high instances of failure is thenatural damaging effect of repeated firings on the arrester unit.

However, and in accordance with the present invention, it has been foundthat another cause for the relatively short lives of many arrestersresults from the processing steps taken in their manufacture.Specifically, high temperature processes are often used to fuse one ofthe pair of carbon electrodes to the insulating spacer. Since copper andother metallic impurities as well as carbon itself can slowly oxidizeunder such extreme conditions, the carbon particles in the electrodetend to loosen and give rise to the formation of a carbon dust on theelectrode. Naturally, the presence of loose carbon particles on thefused electrode surface tends to reduce the gap between the spacedelectrodes after assembly and causes arrester failures by short-circuitat low voltages and normal currents. Thus, this general loosening of thecarbon electrode surface vastly reduces the useable life of the finalarrester unit assembled from the electrode.

Accordingly, this invention provides improved manufacturing techniquesto facilitate the fusing of carbon electrodes to insulating spacers, andat the same time, increase the service life and reliability of carbonelectrode, spark gap type arresters made from the fusedelectrode-insulating spacer components. Until now, the general processused to fuse a carbon electrode to an insulating spacer was carried outby injecting a bonding composition into a suitable bond area between theelectrode and the spacer, heating the electrode, spacer, and bondingcomposition in an air environment until the bonded composition wasliquefied and then cooling the bonding components to solidfy the thebonding composition. The process of this invention, however, eliminatesmany of the difficulties experienced in prior fusing processes bypermitting the fusing of the electrode and spacer to be carried out inan inert atmosphere substantially free from oxygen.

SUMMARY OF THE INVENTION In accordance with the present invention, aprocess for fusing a carbon electrode toan insulating spacer is providedfor use in making a component for electric overvoltage arresters. Thefusing process of this invention in its broadest form involves:transmitting a hollow insulating spacer, inserting a carbon electrodeinto the hollowed protion of the spacer, injecting a measured amount ofa bonding composition in the space between the insulating spacer and theelectrode, heating the spacer, electrode and bonding composition in aninert atmosphere substantially free from oxygen until the bondingcomposition is liquefied, setting the position of electrode in theinsulating spacer while the bonding composition is in a liquid conditionand allowing the bonding composition to harden so that the electrode ispermanently secured within the insulating spacer.

By the process of this invention, a carbon electrode is fused with aninsulating spacer without oxidizing or causing powder formation on thesurface of the electrode. This invention, therefore, allows the fusedcarbon electrode and insulating spacer to be used as a highly reliableunit in electric overvoltage arresters, and still more advantageously,permits the construction of overvoltage arresters having a high repeatedfiring capability. Moreover, this invention overcomes the difficulty ofelectrode surface breakdown and surface oxidation and providesconsistently uniform, satisfactory bonds between the insulating spacerand electrode by carrying out the fusing process in an inert,substantially oxygen-free environment, such as nitrogen or carbondioxide or alternatively in a reducing atmosphere such as dissociatedammonia.

BRIEF DESCRIPTION OE THE DRAWINGS FIG. 1 is a flow sheet showing indiagrammatic form a continuous process for fusing a carbon electrode andan insulating spacer.

FIG. 2 shows one form of the fused insulating spacer and carbonelectrode produced by the process of this invention.

FIG. 3 shows an assembled electric overvoltage arrester, having ascomponents thereof, the fused insulating spacer and carbon electrodemade by the process of this invention.

DESCRIPTION OF THE PREFERRED IMBODIMENT As shown in the flow diagram ofFIG. 1, the first step in the process of this invention involves thetransmission of hollow insulating spacer 10 onto a nesting measured tosupport the spacer. A vibratory type hopper feeder is normally used toautomatically place a series of insulating spacers into a plurality ofsuch nests, which rest on a rotatable dial table. Rotation of the dialtable is automatically timed by an indexing type drive assembly whichrotates the dial table as each insulating spacer is fed to a nest.

A carbon electrode 11 is next automatically fed to and placed in theinsulating spacer supported by the dial table. The insertion ofelectrode 11 is spacer 10 is likewise timed to correspond with therotation of the dial table by indexing drive assembly.

A measured quantity of bonding composition is then deposited or injectedinto the appropriate space 13 between the electrode and insulatingspacer. Although various bonding compositions can be used in thepractice of this invention, a granular lead borate composition ispreferred because of the consistently high strength bonds it formsbetween the carbon electrode and insulating spacer. It is understood,however, that any thermosetting or thermoplastic resin or other bondingcomposition which satisfactorily fuses the electrode and spacer andwhich does not otherwise interfere with the operation of the electricovervoltage arrester is suitable for use in the practice of thisinvention.

Next the assembled insulating spacer, electrode and bonding compositionare moved into a heating chamber by the rotating dial table. Typically,radio frequency induction heating coils are used to heat the assembly totemperatures sufficient to liquefy the bonding composition. When agranular lead borate bonding composition is used, temperatures in therange of about l,000 to l,500 F are required to liquefy the lead borate.During the heating operation, the atmosphere in the heating chamber ismaintained in a substantially oxygen free state so that neither thecarbon nor the metallic impurities in the electrode can oxidize.Although any inert atmosphere substantially free from oxygen can be usedin the practice of this invention, nitrogen is preferred because of itsrelatively low cost and ready availability. Other inert gases of a typenot readily ionized in an R. F. field may also be used in the practiceof this invention.

After the heat treatment of the insulating spacer, electrode and bondingcomposition in an inert atmosphere, the position of the electrode in thespacer is set while the bonding composition is in a liquefied condition.By carefully setting the position of the electrode in the insulatingspacer, a suitable air gap 21 is defined between the electrode 11 andelectrode 15 in the finished electric overvoltage arrester 16.Typically, the air gap is automatically set by the combined action of aspringloaded plunger, which is seated over the top of spacer andelectrode 11, and gap pins located in the dial table, which are forcedup against the carbon electrode. Of course, the entire gap settingoperation is carried out while the bonding composition is in a liquefiedstate.

Finally, after the gap setting operation is completed, the fusedelectrode and spacer assembly preferably are first cooled and thenremoved from the heating chamber and transported around the dial tablefor a sufficient time to allow the assembly to further cool so that thebonding composition can fully solidify. When the fusing operation iscompleted, the finished electrode and spacer are ejected into acollecting device and subsequently assembled into the finished arresterunit.

In the practice of the preferred process of this invention, the carbonelectrode fused to the insulating spacer is a cylindrical rod-shapedelectrode which has a plurality of grooves cut in the one its basesurface which faces the disc electrode disc electrode 15, likewise isprovided with grid pattern of grooves 17 on its opposed base surfaces.In addition, the insulating spacer normally used in'the arrester is abored ceramic sleeve, which surrounds the cylindrical electrode. Sincethe upper surface 19 of the bored ceramic sleeve bears against the lowersurface of the disc electrode in the finished arrester, the positioningof the cylindrical electrode in the sleeve defines the air gap 21between the cylindrical and disc electrodes. When the fusing operationis carried out in air with a lead borate bonding composition attemperatures of about l,000 F to 1,500 F, the cylindrical electrodesurface is left with lose, unbonded particles which readily deteriorateduring subsequent assembly operations. In particular, the groovedpattern on the base surface of the cylindrical electrode has a tendencyto become loose and easily broken during assembly. However, cylindricalelectrodes fused under the same conditions in inert nitrogen or carbondioxide environments have strong surfaces, which are not easily abradedduring subsequent arrester assembly steps, and, therefore, the carbonelectrode surface pattern is neither loosened nor broken. Accordingly,the service life of arresters using the fused electrode spacercomponents prepared by the process of this invention have a much betterrepeated surge capability and longer life than arresters which usedcomponents fused in an air environment.

I claim:

1. In the process for fusing a first carbon electrode with a hollowinsulating ceramic spacer carrying said first electrode in making acomponent for an electric overvoltage arrester having a second electrodeand an air gap between said first and said second electrodes in saidarresters assemblied form by transmitting said insulating spacer to anesting means for supporting said spacer, inserting said first electrodeinto said insulating spacer, and injecting a measured amount of abonding composition between said insulating spacer and said electrode,the improvement comprising heatin the insulating ceramic spacer and thefirst carbon electro e in an men atmosphere substantially free fromoxygen until said bonding composition is liquefied, determining said airgap by setting the position of said first electrode in said spacer whilethe bonding composition is in a liquid condition, and allowing saidbonding composition to harden so as to permanently secure said firstelectrode within said insulating spacer.

2. The process of claim 1 wherein said insulating spacer carrying saidfirst electrode is heated in a nitrogen atmosphere.

3. The process of claim 1 wherein said insulating spacer carrying saidfirst electrode is heated in an carbon dioxide atmosphere.

4. The process of claim 1 wherein said bonding composition is leadborate.

Hun-n

2. The process of claim 1 wherein said insulating spacer carrying saidfirst electrode is heated in a nitrogen atmosphere.
 3. The process ofclaim 1 wherein said insulating spacer carrying said first electrode isheated in an carbon dioxide atmosphere.
 4. The process of claim 1wherein said bonding composition is lead borate.